Silicide backside contact

ABSTRACT

A semiconductor structure and a method of forming the same are provided. In an embodiment, a semiconductor structure includes a first plurality of channel members over a backside dielectric layer, a second plurality of channel members over the backside dielectric layer, a silicide feature disposed in the backside dielectric layer, and a source/drain feature disposed over the silicide feature and extending between the first plurality of channel members and the second plurality of channel members. The silicide feature extends through an entire depth of the backside dielectric layer.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs. Such scaling down has also increased the complexity ofprocessing and manufacturing ICs.

For example, as integrated circuit (IC) technologies progress towardssmaller technology nodes, multi-gate devices have been introduced toimprove gate control by increasing gate-channel coupling, reducingoff-state current, and reducing short-channel effects (SCEs). Amulti-gate device generally refers to a device having a gate structure,or portion thereof, disposed over more than one side of a channelregion. Fin-like field effect transistors (FinFETs) andmulti-bridge-channel (MBC) transistors are examples of multi-gatedevices that have become popular and promising candidates for highperformance and low leakage applications. A FinFET has an elevatedchannel wrapped by a gate on more than one side (for example, the gatewraps a top and sidewalls of a “fin” of semiconductor material extendingfrom a substrate). An MBC transistor has a gate structure that canextend, partially or fully, around a channel region to provide access tothe channel region on two or more sides. Because its gate structuresurrounds the channel regions, an MBC transistor may also be referred toas a surrounding gate transistor (SGT) or a gate-all-around (GAA)transistor. The channel region of an MBC transistor may be formed fromnanowires, nanosheets, other nanostructures, and/or other suitablestructures. The shapes of the channel region have also given an MBCtransistor alternative names such as a nanosheet transistor or ananowire transistor.

As the dimensions of the multi-gate devices shrink, packing all contactfeatures on one side of a substrate is becoming more and morechallenging. To ease the packing density, it has been proposed to movesome routing features, such as power lines (also referred to as powerrails) to a backside of the substrate. Some processes for formingbackside source/drain contacts may damage the source/drain features.Therefore, while existing backside power rail formation processes may begenerally adequate for their intended purposes, they are notsatisfactory in all aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1 illustrates a flowchart of a method for forming a semiconductordevice having a backside source/drain contact, according to one or moreaspects of the present disclosure.

FIGS. 2-25 illustrate fragmentary cross-sectional views of a workpieceduring a fabrication process according to the method of FIG. 1,according to one or more aspects of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper” and the like, may be used herein for ease of description todescribe one element or feature's relationship to another element(s) orfeature(s) as illustrated in the figures. The spatially relative termsare intended to encompass different orientations of the device in use oroperation in addition to the orientation depicted in the figures. Theapparatus may be otherwise oriented (rotated 90 degrees or at otherorientations) and the spatially relative descriptors used herein maylikewise be interpreted accordingly.

Further, when a number or a range of numbers is described with “about,”“approximate,” and the like, the term is intended to encompass numbersthat are within a reasonable range considering variations thatinherently arise during manufacturing as understood by one of ordinaryskill in the art. For example, the number or range of numbersencompasses a reasonable range including the number described, such aswithin +/−10% of the number described, based on known manufacturingtolerances associated with manufacturing a feature having acharacteristic associated with the number. For example, a material layerhaving a thickness of “about 5 nm” can encompass a dimension range from4.25 nm to 5.75 nm where manufacturing tolerances associated withdepositing the material layer are known to be +/−15% by one of ordinaryskill in the art. Still further, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

The present disclosure is generally related to methods of forming asemiconductor device having a backside source/drain contact, and moreparticularly to methods of forming a backside source/drain contactformed of silicide.

Conventionally, source/drain contacts and gate contacts of transistorson a substrate connect source/drain features of the transistors to aninterconnect structure over a front side of the substrate. As thedimensions of IC devices shrink, the close proximity among thesource/drain contacts and gate contacts may reduce process windows forforming these contacts and may increase parasitic capacitance amongthem. The backside power rail (BPR) structure is a modern solution toease the crowding of contacts. In some conventional processes, after asource/drain feature and a frontside source/drain contact are formed,the substrate is flipped over and a backside contact opening is etchedfrom the back side of the substrate. Because the backside contactopening etches and exposes the source/drain feature, the formation ofthe backside contact opening involves risks of damaging the source/drainfeature. Damages of the source/drain feature may increase contactresistance and may undesirably release needed strain exerted on thechannel members.

The present disclosure provides a method for forming a backsidesource/drain contact for MBC transistors. In an example method, aworkpiece is provided. The workpiece includes a fin-shaped structuredisposed over a substrate and a dummy gate stack disposed over a channelregion of the fin-shaped structure. Using the dummy gate stack as anetch mask, a source region and a drain region of the fin-shapedstructure is recessed to form a source opening and a drain opening. Thesource opening is selectively extended further into the substrate toform an extended source opening. A semiconductor plug is deposited intothe extended source opening and a source feature is deposited over thesemiconductor plug. After the formation of the source feature, drainfeature is deposited in the source opening. The workpiece is thenflipped up-side-down with its back side facing up. After the back sideof the substrate is planarized to expose the semiconductor plug, a hardmask feature is formed over the exposed semiconductor plug. Thesubstrate is then removed and replaced with a backside dielectric layer.A silicide precursor is then deposited over the semiconductor plug andthe backside dielectric layer. An anneal process is them performed tobring about silicidation reaction between the silicide precursor and thesemiconductor plug. The silicidation reaction converts the semiconductorplug into a backside source contact that is formed of metal silicide.Because the formation of the backside source contact of the presentdisclosure does not require removal of the semiconductor plug andexposure of the source feature from the back side, there is no riskassociated with damaging the source feature.

The various aspects of the present disclosure will now be described inmore detail with reference to the figures. In that regard, FIG. 1 is aflowchart illustrating method 100 of forming a semiconductor deviceaccording to embodiments of the present disclosure. Method 100 is merelyan example and is not intended to limit the present disclosure to whatis explicitly illustrated in method 100. Additional steps may beprovided before, during and after the method 100, and some stepsdescribed can be replaced, eliminated, or moved around for additionalembodiments of the method. Not all steps are described herein in detailfor reasons of simplicity. Method 100 is described below in conjunctionwith FIGS. 2-25, which are fragmentary cross-sectional views of aworkpiece 200 at different stages of fabrication according toembodiments of method 100. Because the workpiece 200 will be fabricatedinto a semiconductor device 200 upon conclusion of the fabricationprocesses, the workpiece 200 may be referred to as the semiconductordevice 200 as the context requires. Additionally, throughout the presentapplication, like reference numerals denote like features, unlessotherwise excepted.

Referring to FIGS. 1 and 2, method 100 includes a block 102 where aworkpiece 200 is received. In the depicted embodiment, the workpiece 200includes a substrate 202 and a fin-shaped structure 204 disposed overthe substrate 202. The fin-shaped structure 204 extends lengthwise alongthe X direction and is divided into channel regions 204C, source regions204S, and drain regions 204D. In FIG. 2, the workpiece 200 also includesdummy gate stacks 210 disposed over channel regions 204C of thefin-shaped structure 204. Two dummy gate stacks 210 are shown in FIG. 2but the workpiece 200 may include more dummy gate stacks 210. Thesubstrate 202 may be a semiconductor substrate such as a silicon (Si)substrate. The substrate 202 may also include other semiconductormaterials such as germanium (Ge), silicon carbide (SiC), silicongermanium (SiGe), or diamond. The fin-shaped structure 204 may be formedfrom a portion of the substrate 202 and a vertical stack of alternatingsemiconductor layers using a combination of lithography and etch steps.In some instances, the patterning of the fin-shaped structure 204 may beperformed using double-patterning or multi-patterning processes tocreate patterns having pitches smaller than what is otherwise obtainableusing a single, direct photolithography process. The etching process caninclude dry etching, wet etching, reactive ion etching (RIE), and/orother suitable processes. In the depicted embodiments, the verticalstack of alternating semiconductor layers may include a plurality ofchannel layers 208 and a plurality of sacrificial layers 206. Theplurality of channel layers 208 are interleaved by the plurality ofsacrificial layers 206. In some embodiments, the plurality of channellayers 208 may include silicon (Si) and the plurality of sacrificiallayers 206 may be formed of silicon germanium (SiGe). The channel layers208 and the sacrificial layers 206 may be epitaxially deposited on thesubstrate 202 using molecular beam epitaxy (MBE), vapor-phase epitaxy(VPE), ultra-high vacuum CVD (UHV-CVD), and/or other suitable epitaxialgrowth processes.

While not explicitly shown in FIG. 2, an isolation feature 203 (shown inFIG. 23) is also formed around the fin-shaped structure 204 to isolatethe fin-shaped structure 204 from an adjacent fin-shaped structure. Insome embodiments, the isolation feature is deposited in trenches thatdefine the fin-shaped structure 204. Such trenches may extend throughthe channel layers 208 and sacrificial layers 206 and terminate in thesubstrate 202. The isolation feature may also be referred to as ashallow trench isolation (STI) feature. In an example process, adielectric material for the isolation feature is deposited over theworkpiece 200 using CVD, subatmospheric CVD (SACVD), flowable CVD,physical vapor deposition (PVD), spin-on coating, and/or other suitableprocess. Then the deposited dielectric material is planarized andrecessed until the fin-shaped structure 204 rises above the isolationfeature. The dielectric material for the isolation feature may includesilicon oxide, silicon oxynitride, fluorine-doped silicate glass (FSG),a low-k dielectric, combinations thereof, and/or other suitablematerials.

In some embodiments, a gate replacement process (or gate-last process)is adopted where the dummy gate stacks 210 serve as placeholders forfunctional gate structures. Other processes and configuration arepossible. To form the dummy gate stacks 210, a dummy dielectric layer211, a dummy gate electrode layer 212, and a gate-top hard mask layer215 are deposited over the workpiece 200. The deposition of these layersmay include use of low-pressure CVD (LPCVD), CVD, plasma-enhanced CVD(PECVD), PVD, thermal oxidation, e-beam evaporation, or other suitabledeposition techniques, or combinations thereof. The dummy dielectriclayer 211 may include silicon oxide. The dummy gate electrode layer 212may include polysilicon. The gate-top hard mask layer 215 may be amulti-layer that includes a silicon oxide layer 213 and silicon nitridelayer 214. Using photolithography and etching processes, the gate-tophard mask layer 215 is patterned. The photolithography process mayinclude photoresist coating (e.g., spin-on coating), soft baking, maskaligning, exposure, post-exposure baking, photoresist developing,rinsing, drying (e.g., spin-drying and/or hard baking), other suitablelithography techniques, and/or combinations thereof. The etching processmay include dry etching (e.g., RIE etching), wet etching, and/or otheretching methods. Thereafter, using the patterned gate-top hard mask 215as the etch mask, the dummy dielectric layer 211 and the dummy gateelectrode layer 212 are then etched to form the dummy gate stack 210. Asshown in FIG. 2, portions of the fin-shaped structure 204 underlying thedummy gate stacks 210 are channel region 204C. The channel regions 204Cand the dummy gate stack 210 also define source regions 204S and drainregions 204D that are not vertically overlapped by the dummy gate stacks210. Each of the channel regions 204C is disposed between a sourceregion 204S and a drain region 204D along the X direction.

As shown in FIG. 2, the workpiece 200 also includes a gate spacer layer216 disposed along sidewalls of the dummy gate stacks 210 and topsurfaces of the fin-shaped structure 204. In some embodiments, theformation of the gate spacer layer 216 includes conformal deposition ofone or more dielectric layers over the workpiece 200. In an exampleprocess, the one or more dielectric layers for the gate spacer layer 216are deposited using CVD, SACVD, or ALD. The one or more dielectriclayers may include silicon oxide, silicon nitride, silicon carbide,silicon oxynitride, silicon carbonitride, silicon oxycarbide, siliconoxycarbonitride, and/or combinations thereof.

Referring to FIGS. 1 and 3, method 100 includes a block 104 where asource region 204S and a drain region 204D of the fin-shaped structure204 are recessed to form a source opening 222S and a drain opening 222D.After the deposition of the gate spacer layer 216, the dummy gate stacks210 and the gate spacer layer 216 along sidewalls of the dummy gatestacks serve as an etch mask in an etch process that anisotropicallyetches the source regions 204S and the drain regions 204D of thefin-shaped structure 204. The anisotropic etching of the source regions204S and the drain regions 204D results in source openings 222S anddrain openings 222D, respectively. The etch process at block 104 may bea dry etch process or a suitable etch process. An example dry etchprocess may implement an oxygen-containing gas, hydrogen, afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), achlorine-containing gas (e.g., C₁₂, CHCl₃, CCl₄, and/or BCl₃), abromine-containing gas (e.g., HBr and/or CHBr₃), an iodine-containinggas, other suitable gases and/or plasmas, and/or combinations thereof.In embodiments represented in FIG. 3, the source openings 222S and thedrain openings 222D extend through vertical stack of channel layers 208and sacrificial layers 206. In some implementations not explicitly shownin the figures, the source openings 222S and the drain openings 222D maypartially extend into the substrate 202. Sidewalls of the channel layers208 and the sacrificial layers 206 are exposed in the source openings222S and the drain openings 222D.

Referring to FIGS. 1 and 3, method 100 includes a block 106 where innerspacer features 218 are formed. After the formation of the sourceopenings 222S and the drain openings 222D, the sacrificial layers 206exposed in the source openings 222S and the drain openings 222D areselectively and partially recessed to form inner spacer recesses (beingfilled with the inner spacer features 218 in FIG. 3), while the exposedchannel layers 208 are substantially unetched. In an embodiment wherethe channel layers 208 consist essentially of silicon (Si) andsacrificial layers 206 consist essentially of silicon germanium (SiGe),the selective and partial recess of the sacrificial layers 206 mayinclude use of a selective isotropic etching process (e.g., a selectivedry etching process or a selective wet etching process), and the extentat which the sacrificial layers 206 are recessed is controlled byduration of the etching process. The selective dry etching process mayinclude use of one or more fluorine-based etchants, such as fluorine gasor hydrofluorocarbons. The selective wet etching process may include anAPM etch (e.g., ammonia hydroxide-hydrogen peroxide-water mixture).After the formation of the inner spacer recesses, an inner spacermaterial layer is deposited over the workpiece 200, including in theinner spacer recesses. The inner spacer material layer may includesilicon oxide, silicon nitride, silicon oxycarbide, siliconoxycarbonitride, silicon carbonitride, metal nitride, or a suitabledielectric material. The deposited inner spacer material layer is thenetched back to remove excess inner spacer material layer over sidewallsof the channel layers 208, thereby forming the inner spacer features 218as shown in FIG. 3. In some embodiments, the etch back process at block106 may be a dry etch process that includes use of an oxygen-containinggas, hydrogen, nitrogen, a fluorine-containing gas (e.g., CF₄, SF₆,CH₂F₂, CHF₃, and/or C₂F₆), a chlorine-containing gas (e.g., C₁₂, CHCl₃,CCl₄, and/or BCl₃), a bromine-containing gas (e.g., HBr and/or CHBr₃),an iodine-containing gas (e.g., CF₃I), other suitable gases and/orplasmas, and/or combinations thereof.

Referring to FIGS. 1 and 4, method 100 includes a block 108 where thesource opening 222S is selectively extended into the substrate 202 toform an extended source opening 2220. At block 108, a first mask film220 is formed over the workpiece 200, as shown in FIG. 4. The first maskfilm 220 may be a hard mask layer. The first mask film 220 may includesilicon oxide, silicon nitride, silicon oxynitride, siliconoxycarbonitride, silicon carbide, or silicon oxycarbide. In an exampleprocess, a dielectric material is deposited over the workpiece using CVDor ALD to form the first mask film 220 and then a photoresist layer isdeposited over the first mask film 220 using spin-on coating or asuitable process. The photoresist layer is patterned usingphotolithography processes to form a patterned photoresist layer. Thepatterned photoresist layer is then applied as an etch mask in an etchprocess to pattern the first mask film 220. As shown in FIG. 4, thepatterned first mask film 220 cover/protect the drain openings 222Dwhile the source opening 222S is exposed. An anisotropic etch process isthen performed to extend the source opening 222S further into thesubstrate 202 to form an extended source opening 2220. In someinstances, the extended source opening 2220 may extend between about 15nanometer (nm) and about 35 nm into the substrate 202. In someimplementations, the anisotropic etch process at block 108 may be a dryetch process that uses an oxygen-containing gas, hydrogen, afluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃, and/or C₂F₆), achlorine-containing gas (e.g., C₁₂, CHCl₃, CCl₄, and/or BCl₃), abromine-containing gas (e.g., HBr and/or CHBR₃), an iodine-containinggas, other suitable gases and/or plasmas, and/or combinations thereof.

Referring to FIGS. 1 and 5, method 100 includes a block 110 where asemiconductor plug 224 is formed in the extended source opening 2220.With the first mask film 220 still covering sidewalls of the drainopenings 222D, a semiconductor material for the semiconductor plug 224may be deposited in the extended source opening 2220 using molecularbeam epitaxy (MBE), vapor-phase epitaxy (VPE), ultra-high vacuum CVD(UHV-CVD)), and/or other suitable epitaxial growth processes. In anexample process, the semiconductor material may be deposited not only onthe exposed surfaces of the substrate 202 in the extended source opening2220 but also on exposed sidewalls of the channel layers 208. An etchback process may then be performed to remove the semiconductor materialdeposited on sidewalls of the channel layers 208 to form thesemiconductor plug 224, as illustrated in FIG. 5. While the top surfaceof the semiconductor plug 224 is shown as being flat, it may be concaveas a result of the etch back process. The etch back process may includea dry etch process, a wet etch process, or a combination of both. Insome embodiments, the etch back process may be performed such that a topsurface of the semiconductor plug 224 is lower than the top surface ofthe substrate 202 along the Z direction. The semiconductor plug 224 maybe formed of silicon germanium (SiGe). To provide etch selectivity tothe semiconductor plug 224, the semiconductor plug 224 may have a highergermanium concentration than the source feature 230 (shown in FIG. 7, tobe described below). For example, when an n-type MBC transistor isdesired, the source feature 230 is formed of silicon and issubstantially free of germanium (Si) while the semiconductor plug 224 isformed of silicon germanium (SiGe) with between about 15% and 45% ofgermanium (Ge). When a p-type MBC transistor is desired, the sourcefeature 230 is formed of silicon germanium (SiGe) with between about 15%and about 30% of germanium (Ge) while the semiconductor plug 224 isformed of silicon germanium (SiGe) with between about 35% and 45% ofgermanium (Ge).

Referring to FIGS. 1, 6 and 7, method 100 includes a block 112 where asource features 230 are formed in the extended source opening 2220 to bein contact with sidewalls of the channel layers 208. In someembodiments, the source feature 230 includes a first epitaxial layer 226and a second epitaxial layer 228 disposed on the first epitaxial layer226. Because the second epitaxial layer 228 is spaced apart from thesidewalls of the channel layers 208 by the first epitaxial layer 226,the first epitaxial layer 226 may also be referred to as an outer layer226 and the second epitaxial layer 228 may also be referred to as aninner layer 228. Reference is first made to FIG. 6. In some embodiments,the first epitaxial layer 226 may be deposited using an epitaxialdeposition process, such as VPE, UHV-CVD, MBE, and/or other suitableprocesses. The epitaxial growth process may use gaseous and/or liquidprecursors, which selectively interact with the semiconductorcomposition of the channel layers 208 and semiconductor plug 224. Thatis, the first epitaxial layer 226 is deposited on exposed surfaces ofthe channel layers 208 and the surface of the semiconductor plug 224. Insome instances, overgrowth of the first epitaxial layer 226 may extendover the inner spacer features 218. As a result, the first epitaxiallayer 226 may come in direct contact with the inner spacer features 218.The first epitaxial layer 226 is therefore coupled to the channel layers208. Reference is then made to FIG. 7. A second epitaxial layer 228 isthen deposited over the first epitaxial layer 226 using an epitaxialdeposition process, such as VPE, UHV-CVD, MBE, and/or other suitableprocesses. The epitaxial growth process may use gaseous and/or liquidprecursors, which selectively interact with the semiconductorcomposition of the first epitaxial layer 226. In the depictedembodiment, the second epitaxial layer 228 may be spaced apart from thechannel layers 208 by the first epitaxial layer 226. The first epitaxiallayer 226 and the second epitaxial layer 228 may be collectivelyreferred to as the source feature 230.

Depending on the conductivity type of the to-be-formed MBC transistor,the first epitaxial layer 226 and the second epitaxial layer 228 may ben-type or p-type. Example n-type epitaxial layers may include silicon(Si), phosphorus-doped silicon (Si:P), arsenic-doped silicon (Si:As),antimony-doped silicon (Si:Sb), or other suitable material and may bein-situ doped during the epitaxial process by introducing an n-typedopant, such as phosphorus (P), arsenic (As), or antimony (Sb). Examplep-type epitaxial layers may include germanium (Ge), gallium-dopedsilicon germanium (SiGe:Ga), boron-doped silicon germanium (SiGe:B), orother suitable material and may be in-situ doped during the epitaxialprocess by introducing a p-type dopant, such as boron (B) or gallium(Ga). While the first epitaxial layer 226 and the second epitaxial layer228 of a given MBC transistor are of the same conductivity type, theymay have different doping concentrations to reduce contact resistance.For example, when an n-type MBC transistor is desired, the firstepitaxial layer 226 and the second epitaxial layer 228 may includephosphorus-doped silicon (Si:P) and a phosphorus (P) dopingconcentration in the second epitaxial layer 228 is greater than aphosphorus (P) doping concentration in the first epitaxial layer 226.When a p-type MBC transistor is desired, the first epitaxial layer 226and the second epitaxial layer 228 may include boron-doped silicongermanium (SiGe:B) and a boron (B) doping concentration in the secondepitaxial layer 228 is greater than a boron (B) doping concentration inthe first epitaxial layer 226.

As described above, the source feature 230, particularly, the firstepitaxial layer 226, has a smaller germanium concentration than thesemiconductor plug 224. For example, when an n-type MBC transistor isdesired, the first epitaxial layer 226 is formed of silicon and issubstantially free of germanium (Si) while the semiconductor plug 224 isformed of silicon germanium (SiGe) with between about 15% and 45% ofgermanium (Ge). When a p-type MBC transistor is desired, the firstepitaxial layer 226 is formed of silicon germanium (SiGe) with betweenabout 15% and about 30% of germanium (Ge) while the semiconductor plug224 is formed of silicon germanium (SiGe) with between about 35% and 45%of germanium (Ge).

Referring to FIGS. 1 and 8, method 100 includes a block 114 where adrain feature 236 is formed over the drain region 204D. At block 114, asecond mask film 231 is formed over the workpiece 200, as shown in FIG.8. The second mask film 231 may be a patterned hard mask layer. Thesecond mask film 231 may include silicon oxide, silicon nitride, siliconoxynitride, silicon oxycarbonitride, silicon carbide, or siliconoxycarbide. In an example process, a dielectric material is depositedover the workpiece using CVD or ALD to form the second mask film 231 andthen a photoresist layer is deposited over the second mask film 231using spin-on coating or a suitable process. The photoresist layer ispatterned using photolithography processes to form a patternedphotoresist layer. The patterned photoresist layer is then applied as anetch mask in an etch process to pattern the second mask film 231. Asshown in FIG. 8, the patterned second mask film 231 cover/protect thesource feature 230 while the drain opening 222D is exposed.

In some embodiments, the drain feature 236 includes a third epitaxiallayer 232 and a fourth epitaxial layer 234 disposed on the thirdepitaxial layer 232. Because the fourth epitaxial layer 234 is spacedapart from the sidewalls of the channel layers 208 by the thirdepitaxial layer 232, the third epitaxial layer 232 may also be referredto as an outer layer 232 and the fourth epitaxial layer 234 may also bereferred to as an inner layer 234. The formation and composition of thethird epitaxial layer 232 may be similar to those of the first epitaxiallayer 226. In some embodiments, the third epitaxial layer 232 may bedeposited using an epitaxial deposition process, such as VPE, UHV-CVD,MBE, and/or other suitable processes. The epitaxial growth process mayuse gaseous and/or liquid precursors, which selectively interact withthe semiconductor composition of the channel layers 208 and thesubstrate 202. That is, the third epitaxial layer 232 is deposited onexposed surfaces of the channel layers 208 and the surface of thesubstrate 202. In some instances, overgrowth of the third epitaxiallayer 232 may extend over the inner spacer features 218. As a result,the third epitaxial layer 232 may come in direct contact with the innerspacer features 218. The third epitaxial layer 232 is therefore coupledto the channel layers 208. The fourth epitaxial layer 234 is thendeposited over the third epitaxial layer 232 using an epitaxialdeposition process, such as VPE, UHV-CVD, MBE, and/or other suitableprocesses. The epitaxial growth process may use gaseous and/or liquidprecursors, which selectively interact with the semiconductorcomposition of the third epitaxial layer 232. In the depictedembodiment, the fourth epitaxial layer 234 may be spaced apart from thechannel layers 208 by the third epitaxial layer 232. The third epitaxiallayer 232 and the fourth epitaxial layer 234 may be collectivelyreferred to as the drain feature 236.

Depending on the conductivity type of the to-be-formed MBC transistor,the third epitaxial layer 232 and the fourth epitaxial layer 234 may ben-type or p-type. Example n-type epitaxial layers may include silicon(Si), phosphorus-doped silicon (Si:P), arsenic-doped silicon (Si:As),antimony-doped silicon (Si:Sb), or other suitable material and may bein-situ doped during the epitaxial process by introducing an n-typedopant, such as phosphorus (P), arsenic (As), or antimony (Sb). Examplep-type epitaxial layers may include germanium (Ge), gallium-dopedsilicon germanium (SiGe:Ga), boron-doped silicon germanium (SiGe:B), orother suitable material and may be in-situ doped during the epitaxialprocess by introducing a p-type dopant, such as boron (B) or gallium(Ga). While the third epitaxial layer 232 and the fourth epitaxial layer234 of a given MBC transistor are of the same conductivity type, theymay have different doping concentrations to reduce contact resistance.For example, when an n-type MBC transistor is desired, the thirdepitaxial layer 232 and the fourth epitaxial layer 234 may includephosphorus-doped silicon (Si:P) and a phosphorus (P) dopingconcentration in the fourth epitaxial layer 234 is greater than aphosphorus (P) doping concentration in the third epitaxial layer 232.When a p-type MBC transistor is desired, the third epitaxial layer 232and the fourth epitaxial layer 234 may include boron-doped silicongermanium (SiGe:B) and a boron (B) doping concentration in the fourthepitaxial layer 234 is greater than a boron (B) doping concentration inthe third epitaxial layer 232.

Referring to FIGS. 1 and 9, method 100 includes a block 116 where acontact etch stop layer (CESL) 240 and a first interlayer dielectriclayer 242 are deposited. The CESL 240 may include silicon nitride,silicon oxynitride, and/or other materials known in the art and may beformed by ALD, plasma-enhanced chemical vapor deposition (PECVD) processand/or other suitable deposition or oxidation processes. As shown inFIG. 9, the CESL 240 may be deposited on top surfaces of the sourcefeature 230 (including the first epitaxial layer 226 and the secondepitaxial layer 228), the drain features 236 (including the thirdepitaxial layer 232 and the fourth epitaxial layer 234), and sidewallsof the gate spacer layer 216. The first ILD layer 242 is then depositedby a PECVD process or other suitable deposition technique over theworkpiece 200 after the deposition of the CESL 240. The first ILD layer242 may include materials such as tetraethylorthosilicate (TEOS) oxide,un-doped silicate glass, or doped silicon oxide such asborophosphosilicate glass (BPSG), fused silica glass (FSG),phosphosilicate glass (PSG), boron doped silicon glass (BSG), and/orother suitable dielectric materials. In some embodiments, afterformation of the first ILD layer 242, the workpiece 200 may be annealedto improve integrity of the first ILD layer 242.

Referring to FIGS. 1 and 10, method 100 includes a block 118 where thedummy gate stacks 210 are replaced with gate structures 250. To removeexcess materials and to expose top surfaces of the dummy gate stacks210, a planarization process, such a chemical mechanical polishing (CMP)process may be performed to the workpiece 200. With the exposure of thedummy gate stacks 210, block 118 proceeds to removal of the dummy gatestacks 210. The removal of the dummy gate stacks 210 may include one ormore etching processes that are selective to the material in the dummygate stacks 210. For example, the removal of the dummy gate stacks 210may be performed using as a selective wet etch, a selective dry etch, ora combination thereof. After the removal of the dummy gate stacks 210,sidewalls and top surfaces of the channel layers 208 and the sacrificiallayers 206 are exposed in the channel regions 204C. After the removal ofthe dummy gate stacks 210, the sacrificial layers 206 in the channelregions 204C are selectively removed to release the channel layers 208as channel members 2080. In some example processes, the sacrificiallayers 206 may be removed using selective dry etch process or selectivewet etch process. The selective dry etch process may include use of oneor more fluorine-based etchants, such as fluorine gas orhydrofluorocarbons. The selective wet etching process may include an APMetch (e.g., ammonia hydroxide-hydrogen peroxide-water mixture).

Block 118 also includes operations to deposit gate structures 250 in thechannel regions 204C. As shown in FIG. 10, each of the gate structures250 is deposited to wrap around each of the channel members 2080. Eachof the gate structures 250 may include an interfacial layer, a gatedielectric layer over the interfacial layer and a gate electrode layerover the gate dielectric layer. In some embodiments, the interfaciallayer includes silicon oxide and may be formed in a pre-clean process.An example pre-clean process may include use of RCA SC-1 (a mixture ofammonium hydroxide, hydrogen peroxide and water) and/or RCA SC-2 (amixture of hydrochloric acid, hydrogen peroxide and water). The gatedielectric layer may also be referred to a high-k dielectric layer, asit is formed of a dielectric material having a dielectric constantgreater than that of silicon dioxide, which is about 3.9. The gatedielectric layer may be deposited over the interfacial layer using ALD,CVD, and/or other suitable methods. The gate dielectric layer mayinclude hafnium oxide. Alternatively, the gate dielectric layer mayinclude other high-K dielectrics, such as titanium oxide (TiO₂), hafniumzirconium oxide (HfZrO), tantalum oxide (Ta₂O₅), hafnium silicon oxide(HfSiO₄), zirconium oxide (ZrO₂), zirconium silicon oxide (ZrSiO₂),lanthanum oxide (La₂O₃), aluminum oxide (Al₂O₃), zirconium oxide (ZrO),yttrium oxide (Y₂O₃), SrTiO₃ (STO), BaTiO₃ (BTO), BaZrO, hafniumlanthanum oxide (HfLaO), lanthanum silicon oxide (LaSiO), aluminumsilicon oxide (AlSiO), hafnium tantalum oxide (HfTaO), hafnium titaniumoxide (HfTiO), (Ba,Sr)TiO₃ (BST), silicon nitride (SiN), siliconoxynitride (SiON), combinations thereof, or other suitable material.

The gate electrode layer is then deposited over the gate dielectriclayer using ALD, PVD, CVD, e-beam evaporation, or other suitablemethods. The gate electrode layer may include a single layer oralternatively a multi-layer structure, such as various combinations of ametal layer with a selected work function to enhance the deviceperformance (work function metal layer), a liner layer, a wetting layer,an first adhesion layer, a metal alloy or a metal silicide. By way ofexample, the gate electrode layer may include titanium nitride (TiN),titanium aluminum (TiAl), titanium aluminum nitride (TiAlN), tantalumnitride (TaN), tantalum aluminum (TaAl), tantalum aluminum nitride(TaAlN), tantalum aluminum carbide (TaAlC), tantalum carbonitride(TaCN), aluminum (Al), tungsten (W), nickel (Ni), titanium (Ti),ruthenium (Ru), cobalt (Co), platinum (Pt), tantalum carbide (TaC),tantalum silicon nitride (TaSiN), copper (Cu), other refractory metals,or other suitable metal materials or a combination thereof. Further,where the semiconductor device 200 includes n-type MBC transistors andp-type MBC transistors, different gate electrode layers may be formedseparately for n-type MBC transistors and p-type MBC transistors, whichmay include different work function metal layers (e.g., for providingdifferent n-type and p-type work function metal layers). In someinstances, the workpiece 200 may be subject to a CMP process to providea planar top surface.

Referring to FIGS. 1, 11 and 12, method 100 includes a block 120 wherethe workpiece 200 is flipped over and the substrate 202 is planarized.Operations at block 120 may be performed with a back side of theworkpiece 200 facing up. In an example process to flip the workpiece 200over, a carrier substrate is bonded to the front side of the workpiece200 or an interconnect structure (not explicitly shown) formed on thefront side of the workpiece 200. The workpiece 200 is then flipped overalong with the carrier substrate. In some instances, the carriersubstrate may be bonded to the workpiece 200 by fusion bonding, by useof an adhesion layer, or a combination thereof. In some instances, thecarrier substrate may be formed of semiconductor materials (such assilicon), sapphire, glass, polymeric materials, or other suitablematerials. In embodiments where fusion bonding is used, the carriersubstrate includes a bottom oxide layer and the workpiece 200 (or theinterconnect structure thereon, if formed)) includes a top oxide layer.After both the bottom oxide layer and top oxide layer are treated, theyare placed in plush contact with one another for direct bonding at roomtemperature or at an elevated temperature. After the carrier substrateis bonded to the workpiece 200 (or the interconnect structure, ifformed), the workpiece 200 is flipped up-side-down, as representativelyshown in FIG. 11. After the workpiece 200 is flipped over, a back sideof the workpiece 200 is planarized using a CMP process until theisolation feature and the semiconductor plug 224 are exposed, as shownin FIG. 12.

Referring to FIGS. 1 and 13, method 100 includes a block 122 where thesemiconductor plug 224 is etched back. In some embodiments, the etchback at block 122 may be performed using an isotropic etch process. Forexample, the etch back at block 122 may include an APM etch (e.g.,ammonia hydroxide-hydrogen peroxide-water mixture) that etches silicongermanium (SiGe) faster than it does silicon (Si). As shown in FIG. 13,the etch back at block 122 forms a recess 252.

Referring to FIGS. 1, 14, 15, and 16, method 100 includes a block 124where a hard mask feature 256 is formed over the semiconductor plug 224.Operations at block 124 may include a top corner rounding process (shownin FIG. 14), deposition of a hard mask layer 255 (shown in FIG. 15), andetch back of the hard mask layer 255 to form the hard mask feature 256(shown in FIG. 16). Reference is made to FIG. 14. In some embodiments,angled implantation and plasma etching may be performed to round the topcorners of the recess 252, thereby forming a tapered recess 254. Thetapered recess 254 includes sidewalls that taper along the depth of thetapered recess 254 into the substrate 202. The formation of the taperedrecess 254 may be referred to as a top corner rounding process.Referring now to FIG. 15, the hard mask layer 255 is deposited over theback side of the workpiece 200, including over the tapered recess 254and the substrate 202. In some implementations, the hard mask layer 255may be deposited using PECVD, CVD, ALD, PEALD, or a suitable depositionmethod. The hard mask layer 255 may include silicon nitride, siliconcarbonitride, silicon carbide, or metal oxide. The deposited hard masklayer 255 is then etched back until the substrate 202 is exposed, asillustrated in FIG. 16. At the conclusion of the etch back of the hardmask layer 255, the hard mask feature 256 is formed in the taperedrecess 254. The hard mask feature 256 tracks the profile of the taperedrecess 254 and may include tapered sidewalls.

Referring to FIGS. 1, 17 and 18, method 100 includes a block 126 wherethe substrate 202 is replaced with a backside dielectric layer 260. Thereplacement process includes a removal of the substrate 202 anddeposition of the backside dielectric layer 260 in place of the removedsubstrate 202. Referring to FIG. 17, with the hard mask feature 256protecting the semiconductor plug 224, an anisotropic etch process maybe performed to etch away the substrate 202 that is not protected by thehard mask feature 256. An example anisotropic etch process may includeuse of plasma of a fluorine-containing gas (e.g., CF₄, SF₆, CH₂F₂, CHF₃,and/or C₂F₆), a chlorine-containing gas (e.g., C₁₂, CHCl₃, CCl₄, and/orBCl₃), a bromine-containing gas (e.g., HBr and/or CHBR₃), aniodine-containing gas, a halogen fluoride gas mixture, other suitablegases and/or plasmas, and/or combinations thereof. In some instances,the chemistry of the anisotropic etch process is selected such that itetches dielectric material and gate structures 250 at a slower rate. Asshown in FIG. 17, the anisotropic etch process is performed until thegate structures 250 are exposed. In some embodiments, due to the taperedsidewalls of the hard mask feature 256, a sidewall portion 2020 of thesubstrate 202 may remain at the conclusion of the anisotropic etchprocess.

Referring to FIG. 18, after the substrate 202 is removed, the backsidedielectric layer 260 may be deposited over a back side of the workpiece200 by FCVD, CVD, PECVD, spin-on coating, or a suitable process. Thebackside dielectric layer 260 may include silicon oxide or a compositionsimilar to that of the first ILD layer 242. After the formation of thebackside dielectric layer 260, the back side of the workpiece 200 isplanarized by a CMP process to remove the hard mask feature 256 and toexpose surfaces of the sidewall portion 2020 and the semiconductor plug224, as shown in FIG. 18. In some embodiments represented in FIG. 18,before the deposition of the backside dielectric layer 260, a protectiveliner 258 may be deposited over the backside of the workpiece 200,including over the sidewall portion 2020, the gate structures 250, thebottommost inner spacer features 218, and the drain features 236. Insome embodiments, the protective liner 258 may include silicon nitrideor silicon carbonitride and may be deposited using CVD, ALD, or asuitable deposition technique.

Referring now to FIGS. 1 and 19-22, method 100 includes a block 128where the semiconductor plug 224 is converted to a backside sourcecontact 264. Operations at block 128 include performing a pre-silicideimplantation process 300 (shown in FIG. 19), deposition of a silicideprecursor 262 over the workpiece 200 (shown in FIG. 20), performing afirst anneal process 400 (shown in FIG. 21), removal of excess silicideprecursor (shown in FIG. 22), and performing a second anneal process 500(shown in FIG. 22). Referring first to FIG. 19, the pre-silicideimplantation process 300 may implant a semiconductor species, such asgermanium (Ge), in the semiconductor plug 224 and the sidewall portions2020 to provide a more uniform germanium distribution that is conduciveto satisfactory silicide formation. In some embodiments, after thepre-silicide implantation process 300, the workpiece 200 is subject to apre-clean process. The pre-clean process may include use of plasma ofargon (Ar), hydrogen (H₂), ammonia (NH₃), nitrogen trifluoride (NF₃), ora combination thereof. and is aimed to remove undesirable debris fromthe back side of the workpiece 200.

After the pre-silicide implantation process 300 and the pre-cleanprocess, the silicide precursor 262 is deposited over the back side ofthe workpiece 200 to be in contact with the semiconductor plug 224 andthe sidewall portions 2020, as shown in FIG. 20. In some embodiments,the silicide precursor 262 may include a metal that may react withsilicon to form a metal silicide. In some instances, the silicideprecursor 262 may include nickel (Ni), platinum (Pt), or titanium (Ti).In one embodiment, the silicide precursor 262 includes nickel (Ni) dueto nickel's high diffusivity in silicon and the conductive properties ofthe resulting nickel silicide. In some implementations, the silicideprecursor 262 may be deposited using physical vapor deposition (PVD) orchemical vapor deposition (CVD). As shown in FIG. 20, the depositedsilicide precursor 262 is direct contact with the backside dielectriclayer 260, the protective liner 258, the sidewall portion 2020, and thesemiconductor plug 224.

Reference is made to FIG. 21. After the deposition of the silicideprecursor 262, the first anneal process 400 is performed to bring aboutsilicidation reaction between the silicide precursor 262, on the onehand, and the semiconductor plug 224 and the sidewall portion 2020, onthe other hand. In some embodiments, the first anneal process 400 may bea rapid thermal anneal (RTA) process, a laser spike anneal process, aflash anneal process, or a furnace anneal process. The first annealprocess 400 may include an anneal temperature between about 200° C. andabout 300° C. Such an anneal temperature is selected to facilitatediffusion of the silicide precursor 262 into the semiconductor plug 224and the sidewall portion 2020 and to promote silicidation reactionbetween the silicide precursor 262 and the semiconductor plug 224 aswell as the sidewall portion 2020. At the conclusion of the first annealprocess 400, the non-conductive semiconductor plug 224 is converted intothe backside source contact 264, which is electrically conductive. Thebackside source contact 264 may include nickel silicide, platinumsilicide, or titanium silicide, as well as germanium.

Referring to FIG. 22, block 128 also includes removal of the excesssilicide precursor 262 after the first anneal process 400. In someembodiments, the excess silicide precursor 262 may be removed by a wetetch process until the backside dielectric layer 260 and the backsidesource contact 264 are exposed. The wet etch process here may includechemistry that is selective to the silicide precursor 262. In someinstances, the wet etch process may include use of hydrogen peroxide(H₂O₂), hydrofluoric acid (HF), nitric acid (HNO₃), hydrochloric acid(HCl), sulfuric acid (H₂SO₄) or a ferric chloride (FeCl₃) solution. Withthe backside source contact 264 exposed from the back side of theworkpiece 200, a second anneal process 500 is performed to activate thebackside source contact 264 by enriching a more electrically conductivephase of the metal silicide in the backside source contact 264. Forexample, when the backside source contact 264 is formed of nickelsilicide, the second anneal process 500 is performed to enrich the NiSi,Ni₂Si, or NiSi₂ phases that are more electrically conductive (lesselectrically resistive). In some embodiments, the second anneal process500 may be a rapid thermal anneal (RTA) process, a laser spike annealprocess, a flash anneal process, or a furnace anneal process. The secondanneal process 500 may include an anneal temperature greater than theanneal temperature of the first anneal process. In some instances, theanneal temperature of the second anneal process 500 may be between about300° C. and about 400° C.

Referring to FIGS. 1, 23, 24, and 25, method 100 includes a block 130where a backside power rail 266 is formed. FIG. 24 illustrates across-sectional view of the source region 204S along section I-I′ inFIG. 23. FIG. 25 illustrates a cross-sectional view of the workpiece 200in FIG. 23 after the workpiece 200 is flipped over. The backside powerrail 266 may be embedded in an insulation layer 268 shown in FIG. 24. Insome embodiments, the insulation layer 268 may have a compositionsimilar to the first ILD layer 242 and may be deposited over the backside of the workpiece 200, including over the backside dielectric layer260, the protective liner 258, the isolation feature, and the backsidesource contact 264, using spin-on coating, FCVD, or CVD. Then, a powerrail trench may be patterned in the insulation layer 268. A barrierlayer and a metal fill material are then deposited into the power railtrench to form the backside power rail 266. In some embodiments, thebarrier layer in the backside power rail 266 may include titaniumnitride, tantalum nitride, cobalt nitride, nickel nitride, or tungstennitride and the metal fill material in the backside power rail 266 mayinclude titanium (Ti), ruthenium (Ru), copper (Cu), nickel (Ni), cobalt(Co), tungsten (W), tantalum (Ta), or molybdenum (Mo). The barrier layerand the metal fill layer may be deposited using PVD, CVD, ALD, orelectroless plating. A planarization process, such as a CMP process, maybe performed to remove excess materials over the insulation layer. Thebackside power rail 266 is in direct contact with and electricallycoupled to the backside source contact 264, as shown in FIGS. 23, 24 and25.

Reference is still made to FIGS. 23, 24 and 25. In some embodiments,although the sidewalls of the backside source contact 264 may appearsubstantially straight along the protective liner 258, sidewalls of thebackside source contact 264 may balloon or expand sideways into theisolation feature 203. Such sideway expansion is a result of sidewayerosion or etching at block 108 when the extended source opening 2220 isformed. In some instances, the formation of the extended source opening2220 may also etch the isolation feature 203 and expand the extendedsource opening 2220 sideways into the isolation feature 203. Thesemiconductor plug 224 that is deposited into the extended sourceopening 2220 inherits such balloon or sideway-expansion shape. Thesemiconductor plug 224 is later converted into the backside sourcecontact 264. FIGS. 22 and 23 also illustrate how the backside sourcecontact 264 engages adjacent structures. As shown in FIG. 22, thebackside source contact 264 is in direct contact with the protectiveliner 258 but is spaced apart from the backside dielectric layer 260. Asshown in FIG. 23, the backside source contact 264 is in direct contactwith the isolation feature 203. The protective liner 258 does not extendbetween the backside source contact 264 and the isolation feature 203.

Referring to FIG. 25, the source feature 230 includes a first width W1along the X direction and the backside source contact 264 includes asecond width W2 along the X direction. Because the backside sourcecontact 264 is converted from not only the semiconductor plug 224 butalso the sidewall portions 2020, the second width W2 is greater than thefirst width W1. In some instances, the first width W1 may be betweenabout 12 nm and about 16 nm and the second width W2 may be between about16 nm and about 22 nm. In some embodiments represented in FIG. 25, thebackside source contact 264 may come in contact with the bottommost(closer to the backside dielectric layer 260) inner spacer features 218.

Embodiments of the present disclosure provide advantages. Methods of thepresent disclosure form a metal silicide backside source contact withoutthe risk of damaging the source/drain feature. To form a backside sourcecontact using methods of the present disclosure, a semiconductor plug isdeposited in an extended source opening that extends into a substrate ofa workpiece. After the semiconductor plug is exposed from a back side ofthe substrate and the substrate is replaced with a backside dielectriclayer, the semiconductor plug is converted into an electricallyconductive backside source contact. Throughout the process, thesemiconductor plug is not removed and the source/drain feature is notexposed to etchants from the back side. Methods of the presentdisclosure therefore avoid potential damages to the source/drainfeature.

In one exemplary aspect, the present disclosure is directed to asemiconductor device. The semiconductor device includes a firstplurality of channel members over a backside dielectric layer, a secondplurality of channel members over the backside dielectric layer, asilicide feature disposed in the backside dielectric layer, and asource/drain feature disposed over the silicide feature and extendingbetween the first plurality of channel members and the second pluralityof channel members. The silicide feature extends through an entire depthof the backside dielectric layer.

In some embodiments, the semiconductor device may further include abackside metal line underlying the backside dielectric layer. Thesilicide feature extends between the backside metal line and thesource/drain feature. In some implementations, the silicide featureincludes nickel silicide, platinum silicide, or titanium silicide. Insome instances, the semiconductor device may further include adielectric liner disposed between the silicide feature and the backsidedielectric layer. In some embodiments, the dielectric liner includessilicon nitride or silicon carbonitride and the backside dielectriclayer includes silicon oxide. In some embodiments, the semiconductordevice may further include an isolation feature adjacent the backsidedielectric layer. The isolation feature is in direct contact with thesilicide feature. In some instances, the source/drain feature extendsbetween the first plurality of channel members and the second pluralityof channel members along a direction. The silicide feature includes afirst width along the direction and the source/drain feature includes asecond width along the direction. The first width is greater than thesecond width. In some embodiments, the semiconductor device may furtherinclude a plurality of inner spacer features interleaving the firstplurality of channel members. The silicide feature is in contact with abottommost inner spacer feature of the plurality of inner spacerfeatures.

In another exemplary aspect, the present disclosure is directed to asemiconductor structure. The semiconductor structure includes a backsidemetal line, a silicide feature disposed on and in contact with thebackside metal line, a source/drain feature disposed on the silicidefeature, a contact etch stop layer (CESL) disposed on the source/drainfeature, and a dielectric layer disposed over the CESL.

In some embodiments, the silicide feature includes nickel silicide,platinum silicide, or titanium silicide. In some implementations, theCESL includes silicon nitride or silicon carbonitride and the dielectriclayer includes silicon oxide. In some embodiments, the source/drainfeature includes an outer epitaxial layer and an inner epitaxial layerover the outer epitaxial layer. In some implementations, thesource/drain feature extends between a first plurality of channelmembers and a second plurality of channel members along a direction. Insome instances, the silicide feature includes a first width along thedirection, the source/drain feature includes a second width along thedirection, and the first width is greater than the second width. In someembodiments, the silicide feature includes silicon and germanium.

In yet another exemplary aspect, the present disclosure is directed to amethod. The method includes receiving a workpiece that includes afin-shaped structure over a substrate, the fin-shaped structureincluding a plurality of channel layers, and a first dummy gate stackand a second dummy gate stack over the fin-shaped structure. The methodfurther includes forming a source opening in the fin-shaped structurebetween the first dummy gate stack and the second dummy gate stack toexpose sidewalls of the fin-shaped structure, extending the sourceopening into the substrate to form an extended source opening, forming asemiconductor plug into the extended source opening, forming a sourcefeature over the exposed sidewalls of the plurality of channel layersand the semiconductor plug in the extended source opening, planarizingthe substrate to expose the semiconductor plug, after the planarizing,replacing the substrate with a backside dielectric layer, depositing ametal layer over the backside dielectric layer and the exposedsemiconductor plug, and performing an anneal process to bring aboutsilicidation between the metal layer and the exposed semiconductor plug.

In some embodiments, the semiconductor plug includes silicon germanium(SiGe). In some implementations, the metal layer includes nickel,platinum, or titanium. In some instances, the replacing of the substrateincludes etching back the exposed semiconductor plug, forming a hardmask feature over the etched-back semiconductor plug, andanisotropically etching the substrate using the hard mask feature as anetch mask. In some instances, the anisotropically etching leaves behinda portion of the substrate extending along sidewalls of the etched-backsemiconductor plug and the anneal process further brings aboutsilicidation between the metal layer and the portion of the substrate.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a firstplurality of channel members over a backside dielectric layer; a secondplurality of channel members over the backside dielectric layer; asilicide feature disposed in the backside dielectric layer; and asource/drain feature disposed over the silicide feature and extendingbetween the first plurality of channel members and the second pluralityof channel members, wherein the silicide feature extends through anentire depth of the backside dielectric layer.
 2. The semiconductordevice of claim 1, further comprising: a backside metal line underlyingthe backside dielectric layer, wherein the silicide feature extendsbetween the backside metal line and the source/drain feature.
 3. Thesemiconductor device of claim 1, wherein the silicide feature comprisesnickel silicide, platinum silicide, or titanium silicide.
 4. Thesemiconductor device of claim 1, further comprising: a dielectric linerdisposed between the silicide feature and the backside dielectric layer.5. The semiconductor device of claim 4, wherein the dielectric linercomprises silicon nitride or silicon carbonitride, wherein the backsidedielectric layer comprises silicon oxide.
 6. The semiconductor device ofclaim 1, further comprising: an isolation feature adjacent the backsidedielectric layer, wherein the isolation feature is in direct contactwith the silicide feature.
 7. The semiconductor device of claim 1,wherein the source/drain feature extends between the first plurality ofchannel members and the second plurality of channel members along adirection, wherein the silicide feature comprises a first width alongthe direction, wherein the source/drain feature comprises a second widthalong the direction, wherein the first width is greater than the secondwidth.
 8. The semiconductor device of claim 1, further comprising: aplurality of inner spacer features interleaving the first plurality ofchannel members, wherein the silicide feature is in contact with abottommost inner spacer feature of the plurality of inner spacerfeatures.
 9. A semiconductor structure, comprising: a backside metalline; a silicide feature disposed on and in contact with the backsidemetal line; a source/drain feature disposed on the silicide feature; acontact etch stop layer (CESL) disposed on the source/drain feature; anda dielectric layer disposed over the CESL.
 10. The semiconductorstructure of claim 9, wherein the silicide feature comprises nickelsilicide, platinum silicide, or titanium silicide.
 11. The semiconductorstructure of claim 9, wherein the CESL comprises silicon nitride orsilicon carbonitride, wherein the dielectric layer comprises siliconoxide.
 12. The semiconductor structure of claim 9, wherein thesource/drain feature comprises: an outer epitaxial layer; and an innerepitaxial layer over the outer epitaxial layer.
 13. The semiconductorstructure of claim 9, wherein the source/drain feature extends between afirst plurality of channel members and a second plurality of channelmembers along a direction.
 14. The semiconductor structure of claim 13,wherein the silicide feature comprises a first width along thedirection, wherein the source/drain feature comprises a second widthalong the direction, and wherein the first width is greater than thesecond width.
 15. The semiconductor structure of claim 9, wherein thesilicide feature comprises silicon and germanium.
 16. A method,comprising: receiving a workpiece comprising: a fin-shaped structureover a substrate, the fin-shaped structure comprising a plurality ofchannel layers, and a first dummy gate stack and a second dummy gatestack over the fin-shaped structure; forming a source opening in thefin-shaped structure between the first dummy gate stack and the seconddummy gate stack to expose sidewalls of the fin-shaped structure;extending the source opening into the substrate to form an extendedsource opening; forming a semiconductor plug into the extended sourceopening; forming a source feature over the exposed sidewalls of theplurality of channel layers and the semiconductor plug in the extendedsource opening; planarizing the substrate to expose the semiconductorplug; after the planarizing, replacing the substrate with a backsidedielectric layer; depositing a metal layer over the backside dielectriclayer and the exposed semiconductor plug; and performing an annealprocess to bring about silicidation between the metal layer and theexposed semiconductor plug.
 17. The method of claim 16, wherein thesemiconductor plug comprises silicon germanium (SiGe).
 18. The method ofclaim 16, wherein the metal layer comprises nickel, platinum, ortitanium.
 19. The method of claim 16, wherein the replacing of thesubstrate comprises: etching back the exposed semiconductor plug;forming a hard mask feature over the etched-back semiconductor plug; andanisotropically etching the substrate using the hard mask feature as anetch mask.
 20. The method of claim 19, wherein the anisotropicallyetching leaves behind a portion of the substrate extending alongsidewalls of the etched-back semiconductor plug, wherein the annealprocess further brings about silicidation between the metal layer andthe portion of the substrate.